This project will study the functional properties of muscles of the oral region. The relation between load and speed of shortening will be measured using standard physiological techniques for in vivo muscle preparations. These experiments will make it possible to describe, for the first time, the force-velocity relationship of an oral muscle. The digastric muscle of the rabbit is the model used in this work. The adaptability of the oral muscle to changed functional demands will be examined by performing a tenotomy on the digastric muscles. Electromyography and conventional and x-ray motion pictures will be used to study the changes in electrical activity and movement pattern that result from the loss of function of one of the muscle pairs that normally moves the mandible. The tenotomy model will also be used to study adaptation in sarcomere number and regeneration of tendon. The studies of the force-velocity relation and functional adaptation are complementary; the former uses an anesthetized surgical preparation, the latter deals with animals that are unanesthetized and performing normal functions such as chewing. Both approaches contribute to the long-range goal of having a complete understanding of how muscles act to produce normal mandibular movement. This study will add to an already extensive body of knowledge on rabbit jaw function. Advances in clinical orthodontics, orthognathic surgery and treatment of oral muscle dysfunction will rest, in part, on a thorough understanding of normal muscle function.